The invention relates to a radio handset for communication in a radio system allowing communication with a selected base station on a selected channel. In particular the invention relates to a telephone that derives information identifying a local base station by monitoring a signal transmitted thereby.
A radio telephone system typically includes a number of base stations and many radio telephones or handsets. To operate on the system a handset must be able to communicate via a base station on a particular communication channel.
In some systems it is the handset that carries the channel selection algorithms. In such systems it is necessary for the handset to find a local base station and select a free channel when required. A channel carrying information e.g. a traffic or bearer channel, can be monitored to identify a base station in the vicinity.
To enable a channel suitable for traffic to be found when required, a handset typically scans the channels available in the system periodically to have an up-to-date picture of which channels are bearer channels and which are free. The more frequently the channels are scanned, the more up-to-date the system picture held by the handset and the faster on average the handset will be able to select a suitable channel for two-way communication if required. The frequency of scanning has a considerable impact on power consumption and for a battery operated device, battery life.
In a system such as DECT (Digital European Cordless Telecommunications), bearer and free channels are identified by monitoring the Radio Signal Strength Indicator (RSSI) of each channel at regular time intervals. The measured signal strengths are stored in a periodically updated channel list. A high RSSI indicates a bearer channel. Using the RSSI information gathered periodically the base station providing the strongest connection can be identified by determining which base stations are transmitting the strongest signals in the locality. A quiet channel can then be found on a local base station by selecting a channel with a low RSSI. Under the DECT protocol this will typically be the quietest channel on a base station transmitting the strongest signal. The handset must monitor the strongest RSSI signal in order to determine the identity of the base station that is transmitting on that channel.
In order to ensure good performance, the handset needs, in practice to scan for a base station once every 10-20 seconds. This has a considerable impact on power consumption.
In accordance with the invention there is provided a radio telephone for communication in a radio system allowing communication on a selected channel of a selected base station, the telephone being operable to derive information on the system by monitoring a signal transmitted thereby, the telephone comprising processing means operative to scan respective system channels and identify a local base station in response to an indication that a parameter of a monitored signal meets a predetermined criterion and a method of operating a radio telephone to identify local base stations in a system allowing communication on a selected channel of a selected base station, information on the system being derived by the telephone by monitoring a signal transmitted by that system, comprising scanning respective system channels and identifying a local base station responsive to an indication that a parameter of a currently monitored signal meets a predetermined criterion.
In prior art handsets, each time a new channel is required, the most up-to-date scanned information is used to select a channel on the strongest base station. The handset will occasionally scan for a base station with a stronger signal strength. If a stronger base station is found, then the handset will monitor this base station instead and select a channel on that base station (ETSI standard 300 175; 11.3.3., second version 1995). This is a heavy drain on the limited battery power of the handset. By making a scan for a new base station responsive to a parameter of the monitored signal, periodic scans for identifying the strongest base station in the locality can be avoided. This has a beneficial impact on battery life.
If the parameters of the signal transmitted by a particular base station indicate that it is likely to be able to support a traffic channel of reasonable quality, there is no need for the handset to identify another base station. Under these circumstances, the additional scanning time and power consumption required to identify the best local base station rather than the currently monitored adequate base station provides at best only a limited improvement in signal quality.
One suitable measure of the likely quality of a traffic channel on a given base station is the strength of a signal transmitted by that base station and received by handset. A suitable and convenient measure of the strength of a received signal is its RSSI, although other measures such as bit error rate could be utilised. The respective system channels may be scanned to identify a strong signal and transmitting local base station if the RSSI of the monitored signals fall below a predetermined level.
If a traffic channel is already in use by the radio telephone, the trigger RSSI level is preferably set to provide for the possibility of the radio telephone making attempts at locating a suitable channel on successive base stations before the quality of the existing traffic channel deteriorates to an extent that significantly effects data transfer.
Similar considerations ideally apply for initial call set up. The critical concern under these circumstances is finding a suitable traffic channel before the monitored signal is lost. As call set up will be triggered by events other than deterioration of the monitored signal, the RSSI level of the monitored signal needs to be sufficient to indicate that the base station will provide an acceptable traffic channel. The issue of losing data is not so relevant.
In one embodiment of the invention, the radio telephone has the additional capability of assessing the environment of the system it is monitoring and suspending all scanning for additional base stations when a particular environment is detected. This can be particularly advantageous when the system being monitored is a residential, or other system that has only a single base station. Under these conditions the search for additional base stations would only consume power with little benefit.